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Question
How is water purified on a large scale at water works ? Explain with the help of a labelled diagram. Name the substance which is added to kill germs in the drinking water supply ?
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Solution
Various processes are used to purify water on large scale at water works. The source of water for cities is nearby a river or a lake. This water generally contains suspended particles, dissolved impurities and germs. In order to supply this water to homes, it has to be purified. Following steps are involved to purify water at water works:
- In water works, methods like sedimentation, decantation, loading, filtration and chlorination etc., are used to remove undesirable materials from water.
- The source of water supply in a city is either a nearby river or lake (reservoir). From there, it is pumped to a 'sedimentation tank'.
- It is kept here for some time to allow the insoluble substances present in water to settle down at the bottom of the tank.
- It is then sent to a 'loading tank', where some alum is added to water.
- Suspended clay particles in water get loaded here with alum particles. After becoming heavy, they settle down at the bottom of the tank. Thus, the process of loading removes the suspended clay particles from water.
- It is then passed through a 'filtration tank' having three layers, i.e., fine sand layer on the top, coarse sand layer in the middle and gravel layer at the bottom .
- These layers act as filters and remove small suspended particles of the water.
- Water is then passed into a chlorination tank. Here, chlorine is added to water to kill the germs present in it.
- Now the clean and disinfected water is pumped to high storage tanks by pumping stations. From these high storage tanks, water is supplied to homes and factories through a network of big and small pipes.
Figure 65. Supply of drinking water in a city.
Chlorine is added to water to kill germs in the drinking water supply.
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